Here's an interesting helpful link regarding "Cassiar" Junco that might help clarify things. Be sure to read through the comment section too.
http://nwbackyardbirder.blogspot.com/2011/04/not-slate-colored-junco-cassiar-junco.html

Turns out I have an exact same, literally mirror image, of the first image shown on this site in my yard here in Bellevue. And maybe the same bird for the last year or two when I think I erroneously may have dismissed it as Slate-colored. I suspect that there may be far more "Cassiar" records around here than pure "Slate-colored" and there could be a lot of erroneous identifications of the latter which seems like an easy mistake to make. As Greg Gillison notes, "Cassiar" (without the possessive "apostrophe s") is the correct name since the name's origin is geographic referring to an area in far NW British Columbia (mountains and an old asbestos mining ghost town of the same name).

Richard Rowlett
Bellevue (Eastgate), WA